Hi, I found this in Ossett, West Yorkshire, UK and it comes from the carboniferous clay formation.
To me, it looks like a nut inside fossil wood. What do you think?
I think the marks on the outside suggest wood, perhaps a mould (the real thing being absent). I don't think the overall lump is a piece of wood (though you can see all round it better than I can, of course).
The loose/loosish thing inside: I believe that is an internal mould of a nut (such as Trigonocarpus). The nut itself is still present, and we can see a striated internal structure to it. Quite why there is a gap between the inside of the nut's shell and the internal mould, I don't really know; I suspect the gap was originally softer tissues which did not get preserved. But it also looks like the mould has been worn-back somewhat.
The surface of the 'nut' appears to be clay, apart from the darkish surface. I have removed the 'nut' from the possible wood so I can clean it off better. The middle looks very much like a trigonocarpus nut, but I haven't finished cleaning the clay off yet because it takes a while to remove it without damaging it.